1. Field of the Invention
The present invention relates to a power transmitting apparatus for a hybrid vehicle comprising an engine, motors, and power distributors.
2. Description of the Related Art
For example, Japanese Patent Laid-Open No. 11-301291 discloses a known power transmitting apparatus for a hybrid vehicle comprising an engine, motors (electric motors), and power distributors. This hybrid vehicle comprises an engine, two motors, and power distributors (differential gear devices) each composed of two pinion gear devices. The rotational drive power of the engine is distributively inputted to an input shaft of each power distributor via a gear. One of two output shafts of each power distributor is coupled to one of the two motors so that a drive (motoring) or regenerative torque from the motor is applied to the power distributor. Rotative drive powers from the other output shafts of the power distributors are transmitted in parallel to one power output shaft connected to drive wheels of the vehicle. The speed reduction ratio for a rotation transmitting system from the engine to the power output shaft via one of the power distributors is different from the speed reduction ratio for a rotation transmitting system from the engine to the power output shaft via the other power distributor. Specifically, the speed reduction ratio for each rotation transmitting system is determined when one of the two output shafts of the power distributor in the rotation transmitting system which shaft is connected to the motor has a rotation speed of 0.
In the power transmitting apparatus for a hybrid vehicle configured as described above, one of the motors is brought into a driving (motoring) state, the other motor is brought into a regenerative (generating) state, and the motors are set to consume and generate almost equal amount of power. Then, in a steady state (a car speed is almost constant), for the rotation speed ωe and torque Te of the engine and the rotation speed ωv and torque Tv of the power output shaft, the relation Tv=(ωe/ωv)·Te is established. Further, in this case, a speed reduction ratio (ωe/ωv) for the transmission of rotations from the engine to the power output shaft can be changed to an arbitrary value between the two speed reduction ratios of the rotating transmitting system by controlling the torque generated by each motor.
Accordingly, if the vehicle is propelled using the engine as a propulsion source, the speed reduction ratio (speed change ratio) for the transmission of rotations from the engine to the power output shaft can be continuously changed by controlling the torques generated by both motors. This provides functions comparable to those provided if a continuous speed change device such as a CVT is provided between the engine and the power output shaft. That is, the vehicle can be propelled using an output from the engine while changing the speed between the engine and the power output shaft and without the need for a mechanical continuous speed change device such as a CVT.
Furthermore, by causing a deviation in the state in which the power consumption of the motor in the driving state is almost equal to the power generation of the motor in the regenerative state, it is possible to use the motor to generate an assist drive power or to charge a battery that is a power source for the motor. Moreover, by allowing both motors to transmit torques to the power output shaft while controlling the torques of both motors so that the load torque on the engine is zero, the vehicle can be propelled using only the output from the motors (what is called EV propulsion) and without using the output from the engine.
To improve the energy efficiency of a hybrid vehicle of this kind, it is preferable to carry out EV propulsion of using only the output from the motors and without using the output from the engine, during, for example, cruise propulsion in which the vehicle speed is relatively low. This is because when the vehicle is propelled using the output from the engine in a low vehicle speed region, it is generally difficult to operate the engine at an operation point with a high energy efficiency. For the EV propulsion, it is preferable to properly charge the battery using the output from the engine in order to supplement the power of the battery, which is the power source of the motors. Specifically, during, for example, the cruise propulsion at a low vehicle speed, to improve the energy efficiency of the hybrid vehicle, it is preferable to carry out what is called series type EV propulsion in which the vehicle is propelled using the outputs from the motors, while properly utilizing the output from the engine to charge the battery.
However, in the hybrid vehicle disclosed in Japanese Patent Laid-Open No. 11-301291, while the engine is being operated, the output from the engine is always inputted to the power distributors. It is thus impossible to allow one of the motors to generate power using the output from the engine while propelling the vehicle using the drive power of the other motor, regardless (independently) of the propulsive state (desired propulsive torque and speed of the vehicle and the like) of the vehicle. That is, the series type EV propulsion cannot be carried out.
Further, with the invention described in Japanese Patent Laid-Open No. 11-301291, when the EV propulsion is carried out to propel the vehicle using the drive power of the motors and without using the output from the engine, the torques of both motors are transmitted to the power output shaft while offsetting the torque transmitted by each motor to the engine so that the load torque on the engine is zero. Thus, when carrying out the EV propulsion, each motor must generate a large torque. Consequently, it is likely that the capacity of each motor must be increased, and the power loss of each motor is likely to increase.
On the other hand, the applicant has proposed, in Japanese Patent Laid-Open No. 2002-52944, a hybrid vehicle comprising two power distributors and two motors which vehicle is capable of not only the speed change propulsion as in the case of Japanese Patent Laid-Open No. 11-301291 but also the series type EV propulsion or the like. The technique disclosed in Japanese Patent Laid-Open No. 2002-52944 comprises a rotation transmission path through which rotations are transmitted between one of the motors and the engine without using any power distributors and a rotation transmission path through which rotations are transmitted between the other motor and the power output shaft without using any power distributors. A clutch is provided in each of these rotation transmission paths and rotation transmission paths from the respective motors to the corresponding power distributors. Then, the proper combination of connections and disconnections of the clutches enables the vehicle to travel in various propulsive modes including the series type EV propulsion. However, the technique disclosed in Japanese Patent Laid-Open No. 2002-52944 requires more rotation transmission paths and clutches than that disclosed in Japanese Patent Laid-Open No. 11-301291, previously described. Consequently, with the technique disclosed in Japanese Patent Laid-Open No. 2002-52944, it is difficult to reduce the size of the while power transmitting apparatus.
Moreover, with the technique disclosed in Japanese Patent Laid-Open No. 11-301291, if the engine becomes inoperative (the engine does not generate any output) for any reason while carrying out change speed propulsion, then in particular the rotation speed of one of the motors which is connected to the power distributor in a system from the engine to the power output shaft which system has a higher speed reduction ratio for the transmission of rotations becomes high compared to the service rotation speed region during the normal speed change propulsion. Thus, the higher-speed-reduction-ratio motor is desired to endure a rotation speed (which does not occur during the normal propulsion) that is high compared to the service rotation speed region during the speed change propulsion. Further, a drive circuit for this motor must have a sufficiently large conduction capacity compared to the service region during the speed change propulsion. As a result, the sizes of the motors and its drive circuit must be increased, thus preventing miniaturization of the power transmitting apparatus.
In view of this background, it is an object of the present invention to provide a power transmitting apparatus that enables a small configuration to accomplish not only the speed change propulsion but also various other forms of propulsions such as the EV propulsion including the series type EV propulsion. It is another object of the present invention to provide a power transmitting apparatus that can use a simple configuration to prevent motors from rotating at excessively high speed even if an engine becomes inoperative during the speed change propulsion.